This invention relates to a fully solid type lithium ion secondary battery and a method for manufacturing the same.
As an electrolyte in a lithium ion secondary battery, an electrolyte in which a porous film called a separator is impregnated with a non-aqueous electrolytic solution has been generally used. Since this type of electrolyte is likely to cause leakage of liquid or combustion, there has recently been a proposal for using, instead of such electrolyte comprising liquid, a fully solid battery which uses an inorganic solid electrolyte. The fully solid battery which does not use a combustible organic solvent such as an electrolytic solution has no risk of leakage of liquid or combustion and therefore has excellent safety. Since, however, components of the fully solid battery, i.e., a positive electrode, an electrolyte and a negative electrode, are all made of solid substance, it has difficulty in securing sufficient contact in its interfaces between the positive electrode and the electrolyte and the negative electrode and the electrolyte resulting in increase in resistance in the interfaces. In this case, lithium ion conductivity in the interfaces between the electrolyte and the electrodes is not sufficiently high and, for this reason, such fully solid battery has not been offered for practical use yet.
As a method for manufacturing such fully solid battery efficiently, it is conceivable to prepare green sheets of a solid electrolyte and positive and negative electrodes from slurries which respectively comprise powder of specific compositions, and laminate such green sheet of the solid electrolyte, green sheet of the positive electrode and green sheet of the negative electrode together to provide a laminate of a lithium ion secondary battery.
In this case, there is the problem that, since the solid electrolyte, positive electrode and negative electrode are made of materials which differ from one another and have different optimum sintering temperatures from one another, if the green sheets of the solid electrolyte, positive electrode and negative electrode are sintered in their laminated state at a single temperature as a package, a warp or cracking occurs in the completed solid battery and the best properties of the solid electrolyte, positive electrode and negative electrode cannot be demonstrated sufficiently. For overcoming this problem, the materials of the solid electrolyte, positive electrode and negative electrode must be adjusted for obtaining an optimum sintering temperature which is common to these materials and, as a result, limitation is imposed to capacity and output of the battery.
It is, therefore, an object of the invention to provide a method for manufacturing a fully solid type lithium ion secondary battery having sufficient ion conductivity in an efficient manner.